It has been recently proposed a fluidized bed gasification method for gasification of raw material such as coal or biomass, using a circulating fluidized bed furnace with fluidized bed combustion and gasification furnaces which is called twin-towered gasification furnace (see Reference 1).
FIG. 1 shows a circulating fluidized bed furnace according to the Reference 1 which comprises a fluidized bed combustion furnace 1 supplied with air for combustion of char by means of a fluidized bed so as to heat solid particles such as sand (bed material or fluid medium). In the fluidized bed combustion furnace 1, char and solid particles are introduced from below while supplementary fuel F is supplied through a lateral supplementary raw material port 2. The fluidized bed combustion furnace 1 is provided at its bottom with a wind box 4 connected to an air supply line 3 for blowing of air, and at its top with a heat exchanger 5 for heat recovery.
The top of the fluidized bed combustion furnace 1 is connected through a transfer pipe 7 to a separator 6 comprising a cyclone. The separator 6 has outer and inner cylinders 8 and 9, burnt gas (hot fluid) 10 from the fluidized bed combustion furnace 1 being introduced via the transfer pipe 7 tangentially into the outer cylinder 8 where it is centrifuged into solid particles 11 and exhaust gas 12. The exhaust gas 12 with fine-grained ash is discharged through the inner cylinder 9 while the solid particles 11 with rough-grained unburned char is supplied to a fluidized bed gasification furnace 14 via a downcomer 13 extending downward from a lower end of the outer cylinder 8 of the separator 6.
The fluidized bed gasification furnace 14 comprises an introductory portion 15 for introduction of the hot solid particles 11, a gasification portion 18 for gasification of raw material 17 such as coal from a raw material supply device 16 through heat from the solid particles 11, a lower communicating portion 20 for communication between the introductory and gasification portions 15 and 18 at a lower part of the fluidized bed 19 so as to allow the movement of the particles and a gasification agent box portion 21 extending over bottoms of the portions 15, 18 and 20 for supply of the gasification agent such as steam into the fluidized bed gasification furnace 14, the box portion 21 being connected with a gasification agent supply line 22. As shown in FIG. 1, the lower communicating portion 20 within the fluidized bed 19 is in the form of a backflow prevention structure for prevention of backflow of the burnt gas in the fluidized bed combustion furnace 1 into the separator 6.
The char not gasified in the gasification portion 18 and the solid particles are supplied for circulation to the fluidized bed combustion furnace 1 via a supply flow passage 23 comprising for example an overflow pipe, the char being burnt out with the air under a condition of air ratio being 1.2 while the solid particles are heated again by the combustion of the char.
If coal is fed as raw material to be gasified to the gasification portion 18, produced is produced gas 24 mixed with gas components such as hydrogen (H2), carbon monoxide (CO) and methane (CH4); if biomass or the like with a high water content is fed as raw material to be gasified, produced is produced gas 24 with the above-mentioned gas components containing much steam. The produced gas 24 is taken out via a discharge pipe 25 from the fluidized bed gasification furnace 14 into a recovery device 26 where the produced gas 24 is separated from impalpable powder 27 having been entrained in the gas and is derived through an inner pipe 28. The produced gas 24 thus derived may be pressurized and supplied as fuel to, for example, a gas turbine, or may be supplied to a refinery for production of any required gas from the produced gas 24.
[Reference 1] JP2005-41959A